The ants are probably feeding in the tree - perhaps they like the sap your tree is manufacturing. Ants are known to herd aphids, and will sometimes visit scale insects or mealybug colonies to collect honeydew, a sweet substance these insects excrete. Inspect the trunk and branches for splits, cracks or other injuries where sap may be escaping. While you're there, look for colonies of aphids, scales or mealybugs. Once you've determined the source of the food for the ants, you'll be able to treat the injury or pest population to eliminate the food source. Once this is accomplished, the ants will no longer be interested in your tree.